The world craves an effective, risk-free vaccine against recalcitrant foes like AIDS and malaria. Creating mock infections with tiny rings of bacterial DNA may be the answer.

Accidents happen, and in the lab as anywhere else the upshot is usually just exasperation. But every now and then a screwy experimental outcome heralds a major breakthrough. Inadvertencies, after all, brought fluid dynamics to Archimedes, the telephone to Bell, penicillin to Fleming, the Post-it to 3M. And now a comparable experimental anomaly--this one in a California gene-therapy lab--seems to have launched an idea that may well revolutionize the approach to vaccines, medicine’s longest-playing miracle. The key innovation involves a tiny ring of DNA called a plasmid, which is normally found in a bacterium and which in itself is not news: plasmids have long been workhorses in the biotech revolution because they can be tailored to crank out any number of desired proteins. The surprise is that plasmids, with the right genes stitched into them, can also be used to create a mock infection in mice and presumably in humans too. Most important of all, that mock infection stirs up an unmistakably real--and surprisingly effective--immune response.

DNA vaccines excite specialists because they may solve several long-standing immunologic puzzles. Some microbial diseases, like malaria and aids, have proved uncanny in their ability to elude vaccines altogether, while others--like influenza--play a maddening game of cat and mouse, reappearing one year in genetically distinct forms that make them impervious to the previous year’s vaccine. But in the war against such microbial plagues, DNA vaccines just might provide the magic bullet. The experimental results of DNA vaccine studies in animals, including one against hiv in chimps, have been so promising that clinical trials are in the works for a range of diseases--including not just malaria and aids but also flu, herpes, hepatitis B, and even T cell leukemia.

The accident that launched DNA vaccines happened in 1989 at Vical, a San Diego biotech company. Biochemist Philip Felgner and his colleagues were experimenting with plasmids to see if they could be used to insert therapeutic genes into mouse tissue. Mice, of course, are mammals, not bacteria, and the question Felgner wanted to answer was simply whether plasmids could function in mammalian cells. The first step was to package the plasmids in tiny spherical structures called liposomes, which are made up of fatty molecules. These molecules pass easily through cell membranes, and Felgner’s group hoped that the liposomes would ferry the bacterial DNA into mice muscle cells, where it could set up shop and start churning out proteins. As a control, however, they injected naked plasmid DNA--dna, that is, without a liposome sphere around it--into another population of mice. The team expected the DNA to remain inert, since it lacked a vehicle to transport it into the mammalian cells. To their astonishment, however, the plasmid not only got into the mouse tissue but also began sputtering out minute quantities of protein.

We thought we’d switched our test tubes or mislabeled them, Felgner recalls. So then we did an experiment using no lipids at all. We were still getting protein expression and thought maybe lipids were contaminating the experiment. But within a couple of weeks the group had established that plasmids could indeed somehow insinuate themselves into cells and produce small amounts of protein all by themselves.

Felgner, intrigued, then began considering how this novel mechanism might be put to practical use. The plasmids weren’t making enough protein for immediate use in gene therapy. But you don’t need to produce a lot of proteins for vaccines, Felgner points out. In fact, immunologists thought that whatever amount was required to stimulate immunity in a mouse would also work for a man. Or for that matter, Felgner adds, a cow.

What you do need, however, is for the proteins to be introduced to the appropriate cells of the immune system. And muscle cells, although clearly able to take up DNA, weren’t thought to be very good at tipping off the immune system when an invasion of the body was under way. Yet the next set of experiments proved otherwise. Felgner’s group established that when injected into muscle cells a loop of plasmid DNA with the right hiv gene stitched in would not only code for the protein but also raise an immune response against the virus. Felgner brought this work to the attention of Margaret Liu, a director of viral biology research at Merck. The pharmaceutical giant, a leader in vaccine development, would have the resources to launch a full-scale collaborative investigation. Initially dubious, Liu decided to see whether Felgner’s system might arouse immunity against a viral foe like influenza. She added a gene encoding protein from the flu virus to a plasmid and confirmed that it, too, worked brilliantly to raise immunity against influenza in mice.

For at least a millennium, humans have known that even a tiny dose of a disease agent can trip alarms in the immune system and cause it to quickly erect a wall of safeguards against the disease. As early as A.D. 900 the Chinese had devised a secret rite for fending off smallpox that involved grinding up pox scabs and inhaling them (don’t retch, it seems to have worked). The practice made its way to India and, ultimately, to Turkey. There, in 1717, a redoubtable Englishwoman, Lady Mary Wortley Montagu, encountered a somewhat different version that she brought home to Britain. It too worked, but it was risky: some people who tried it came down with full-blown or even fatal cases of the disease.

Finally, in 1796, Edward Jenner improved the procedure dramatically when he discovered that he could safely raise immunity against smallpox by inoculating patients with matter from cowpox, a virus related to smallpox but completely harmless to humans. Cowpox lent its Latin name-- vaccinia--to vaccination, the procedure that, over the next century and a half, gave us once undreamed-of protection against dreaded bacterial diseases including typhoid, cholera, and tetanus, and viral maladies such as rabies, yellow fever, and polio.

DNA vaccines hold the promise of doing the same, but far more safely and effectively. The key new ingredient is the plasmid--a biogadget that performs like a specialized protein factory in bacteria and some fungi. Bacteria swap plasmids like prize baseball cards: in fact it’s one of the most common ways they acquire and pass on the useful genes that enable them to resist antibiotics. The biotech revolution has exploited this specialization. The trick is to insert a desired gene into a plasmid, then tailor the plasmid so that it can produce only that particular gene’s protein.

As Felgner found, plasmids can also be coaxed into working inside the cells of higher animals. Fortunately, these domesticated plasmids don’t mass-reproduce themselves, so we humans needn’t worry about an uncontrolled population explosion of rogue plasmids dumping large amounts of strange proteins into our bodies. But individual plasmids can nonetheless survive even in the highly alien surroundings of mouse or human muscle cells. They don’t splice their genes into our own genome, yet their DNA can crank out small but nonetheless sufficient quantities of immunity- generating protein.

The concept of vaccines containing DNA isn’t new, Liu points out. After all, Jenner’s pioneer smallpox vaccine--made, as it was, out of a live virus--also had DNA in it. The difference here is that the plasmid DNA is carrying just a select gene from the microbe, not the information for producing a live virus. A plasmid can’t run wild, reproducing itself ad infinitum, as a virus or a bacterium might. And plasmids can be tailored to contain only the DNA needed to generate an effective immune response.

To understand why infecting a cell with a single gene from a pathogen can work well enough to stimulate immunity against the whole microbe, you must master a few basic rules in the grammar of the human immune system. Most of us know that when vaccinated for polio, you develop antibodies against the poliovirus and become immune to it. But because of that knowledge, we’ve come to equate antibodies with immunity: a gross and misleading oversimplification. Antibodies aren’t always enough to protect you. People infected with hiv raise a raft of antibodies against the virus yet still succumb to aids.

Why? Over the past couple of decades, researchers have learned that human immunity is an intricate system, with several interdependent yet separate components. Two seem particularly vital: the antibody system and the cell-mediated system. Like an army and an air force, their missions and their equipment aren’t all the same. The antibody system is largely aimed at microbes floating free in your bloodstream, while the cell-mediated system attacks pathogens that have infiltrated your cells.

Antibodies are produced by a class of white blood cells called B cells. Every human inherits a huge library of these cells, including sample populations expressly designed to latch onto foreign proteins. When a virus or bacterium gets into your bloodstream, the system detects its foreign proteins, and the B cells produce large quantities of antibodies that attach themselves to the microbes, marking them for destruction by scavenger cells. Once the antibody system has been activated, cells that recognize the activating proteins remain primed, which speeds up their ability to defend against a second assault. The antibody system has memory, in other words, and that’s important: the foreign protein needn’t persist in your body in order to keep your immune system alert against the pathogen that contains it.

But what about microbes that pass through the bloodstream and slip inside your cells? Viruses are particularly adept at this, in fact they depend on it for their survival. hiv, for example, can hide in a dormant state in your cellular DNA for years. Yet any immune device that indiscriminately attacked your own cells would quickly do more damage than good. This is where the second branch of your body’s natural defenses-- cell-mediated immunity--goes to work.

When a microbe actually makes its way inside one of your cells, the infected cell breaks down some of the virus and combines bits of the viral proteins with its own proteins in a display on the surface of the cell. When the immune system sentinels, known as killer T cells, pass by looking for intruders, they will recognize the telltale viral proteins in the display and kill the infected cell. Finally, just as in the antibody system, the T cells can remember the foreign protein, thus keeping the cell-mediated system on red alert, able to react quickly to renewed invasion.

Unfortunately, a stubborn problem with vaccines is that they don’t always fully activate both arms of the immune system. And therein lies the problem: if a cell-mediated immune response is what you need to ward off a particular disease, a vaccine that primarily stimulates the antibody system may not help.

Traditional vaccines, like Jenner’s against the smallpox virus or later antibacterial vaccines against cholera and tuberculosis, were made from whole microbes that had either been killed or weakened to the point at which they usually couldn’t cause disease. Such vaccines are easily recognized by the antibody system, but they’re not always so efficient at getting into enough cells to activate the cell-mediated system. On the other hand, if they do enter cells, they’re potentially dangerous: What if even a few of the supposedly dead or weakened microbes turn out to be virulent? In an aids vaccine using whole virus, even a few footloose viral particles might get into your cells and initiate a fatal infection. This isn’t an idle fear. In the 1950s a defective batch of Jonas Salk’s polio vaccine left the lab containing live virus. Ultimately it caused 260 cases of polio, 11 deaths, and a wave of panic among the parents of newly immunized children.

Some traditional vaccines (like the one for tetanus) circumvent such dangers by using only proteins derived from the pathogen. And modern biotechnology has refined this principle enormously. A gene coding for some immunity-arousing protein can be snatched out of a pathogen and implanted in bacteria, which then go on to mass-produce it. A hepatitis B vaccine introduced in 1986 was the first vaccine to use proteins derived in this manner. Such protein-based vaccines, of course, can’t accidentally start an infection, and neither can DNA vaccines.

But DNA vaccines have more than just safety going for them. They’re also particularly powerful at stimulating cell-mediated immunity. That’s because the plasmid DNA produces a protein that infected cells display on their surface; that display, in turn, stimulates the permanent proliferation of killer T cells--bullets with the pathogen’s name inscribed on them. You might call it all-natural immunology, since DNA vaccines call up a cell-based immune response, yet without the potential risk a dead or weakened whole virus presents.

DNA vaccines offer still another advantage. One of the perennial frustrations of flu vaccines is that they have to be continually reformulated to fight new or long-dormant strains of influenza virus that sweep across the country each season. That can lead to trouble, as happened in 1976, when a new and apparently dangerous strain of swine flu appeared at Fort Dix, New Jersey. The government and the pharmaceutical industry rushed to develop a new killed-virus vaccine, sped it into production, rapidly immunized 45 million people--and created a major medical disaster. It turned out that the vaccine could sometimes trigger a debilitating and potentially lethal neuromuscular disease, Guillain-Barré syndrome. And to make matters worse, the threatened swine flu epidemic never happened, making the vaccine pointless as well as sometimes dangerous.

But the ability to fine-tune the contents of DNA vaccines makes such debacles highly unlikely. Instead of relying on proteins found on the virus’s outer envelope (essentially, a virus is simply some genetic material wrapped in a protein coat), which tend to vary from strain to strain, a DNA vaccine might feature a gene for a virtually never-changing protein concealed deep inside the virus and common to all known strains. When Liu and her colleagues first tried out a DNA flu vaccine on mice, they designed a plasmid that carried a gene for an interior protein of a flu strain dating back to the 1930s--and found that it raised immunity in the mice against a flu strain that hadn’t arisen until the late 1960s.

By employing such a strategy, it might be possible to use several plasmids--each bearing a gene for a particular viral protein--to come up with a vaccine that arouses an effective cell-mediated response against hitherto resistant diseases. Cell-mediated immunity is thought to be a particularly vital defense against aids, for example. Thus the DNA vaccine that protected chimps from hiv infection carried two genes encoding internal viral proteins and one gene encoding a viral-envelope protein.

hiv is the most prominent example, Liu says, but other diseases include tuberculosis, where antibody response may not be useful, or certain parasitic diseases like malaria. These diseases all involve pathogens that sneak inside cells, where the antibody system can’t see them.

Pathogens are not the only possible target for DNA vaccines. Some researchers have proposed DNA vaccines against cancer, allergies, and perhaps even against autoimmune conditions like multiple sclerosis, juvenile diabetes, and rheumatoid arthritis. The idea is to use plasmid- borne genes to churn out proteins within a cell; when the proteins appear at the cell surface, they will tip the balance of the immune response in favor of either attacking or tolerating a cell bearing a particular protein. In an allergic reaction to dust mites, for instance, the immune system overreacts to the presence of some foreign protein. In an immunologic twist that isn’t fully understood, providing more of that protein at the cell surface might promote tolerance by immune system soldiers, persuading them to call off the attack. Paradoxically, in the case of cancer, it may be possible to use plasmid-borne genes to boost immune recognition--and attack--of a particular protein displayed only on cancer cells.

Besides the panoply of possible applications, one of the most compelling virtues of DNA vaccines is simple economics. The key to effective vaccines is affordability, and plasmids are relatively simple and easily designed: a DNA vaccine might contain plasmids carrying genes coding for effective responses against multiple strains of, say, hiv or other rapidly mutating pathogens--or even multiple diseases. And finally, Liu adds, dna may be more stable as a vaccine entity than current ones. For example, we may not need to freeze or refrigerate it, which would make it a godsend in emergencies or in Third World countries.

We’ve gotten used to thinking of DNA as the key to life; we’ve even begun to adjust to the idea that it can be manipulated to reproduce life or change it. Now we’ve turned up a talent in bacterial DNA that promises to protect life, too. The question is whether DNA vaccines can live up to their promise in humans.